Apparatus and method for wet cleaning or etching a flat substrate

ABSTRACT

An apparatus for wet cleaning or etching of flat substrates comprising a tank with an inlet opening and outlet opening for said substrates. Said tank contains a cleaning liquid and is installed in a gaseous environment. At least one of the openings is a slice in a sidewall of the tank and is present below the liquid-surface. In the tank there may be a portion above the liquid filled with a gas with a pressure being lower than the pressure within said environment. The method comprises the step of transferring a substrate through the cleaning or etching liquid at a level underneath the surface of said liquid making use of said apparatus.

This is a divisional of application Ser. No. 08/881,680 filed Jun. 24,1997 now U.S. Pat. No. 6,247,481 which claims benefit of provisional ofNo. 60/020,060 filed Jun. 24, 1996 which also claims benefit of No.60/020,373 filed Jun. 25, 1996.

FIELD OF THE INVENTION

The present invention is related to the fabrication of flat substratessuch as semiconductor wafers, flat panel displays, solar cells formicroelectronics applications or of flat substrates for otherapplications which require a cleaning or etching step of said substratesduring manufacturing.

The present invention relates more particularly to an apparatus and amethod for wet cleaning or etching of said flat substrate.

BACKGROUND OF THE INVENTION

In the processing of semiconductor wafers, the semiconductor surfacepreparation prior to various processing steps such as oxidation,deposition or growth processes, has become one of the most criticalissues. With the rapid approach of subhalf micron design rules, verysmall particles with submicron dimensions and low levels of metalimpurities (−10¹⁰ atoms/cm²) can have a detrimental effect on processyields. A commonly used technique to reduce foreign particulate mattercontamination level on semiconductor surfaces, is the immersion ofwafers in liquids such as chemical solutions. The semiconductor wafersare usually placed into a tank or vessel or wet bench, which holds aliquid for cleaning or etching. Such tank or vessel or wet bench isinstalled in a clean room environment. Often a wet bench containsseveral vessels or tanks.

Semiconductor wafers or substrates can be immersed into the liquid fromthe upper part of the tank. Another way to put the substrates in contactwith the liquid is to use a door or an entrance at the side wall of thetank and to place a batch of wafers into the tank. After the door isclosed and sealed, liquid is flowing into the tank, again to makecontact between the cleaning or etching liquid and the substrates. Inboth approaches, one can switch from one liquid to the other liquid inthe same tank. This method is commonly denominated as a single tankprocessor. Variations of these two techniques are also used for singlewafer cleaning or etching purposes. In such a case, the wafer can beplaced into a small vessel, which can be larger than the wafer. In somecases, the wafers are immersed into the liquid; in other cases, thewafers are loaded into an empty vessel and the liquid is flowing intothe vessel.

A particular apparatus used after cleaning or etching of semiconductorwafers is disclosed in EP 0 385 536, the teaching of which isincorporated herein by reference.

EP-A-407-044 discloses a system having two rooms, each containing aliquid. The system can be used in a semiconductor wafer manufacturingline and allows a wafer to be transported from one room of the system toanother room of the system without an intermixing of the liquidsoccurring. The level of the liquids in the two rooms is adapted as toadjust the respective pressures of the liquids and an elaborate shutterdevice is provided in the system for opening and closing an openingbetween the two rooms without causing intermixing through flow andturbulence of the liquids.

A first drawback of the above-mentioned tools and methods is thefootprint necessary, i.e., the surface area within a clean room occupiedby the tank or vessel. Above-mentioned tools making use of a tank orvessel are big and occupy a large area within a clean room. It is to beunderstood that clean room area is very expensive due to the costs ofmaintaining an ultra-clean atmosphere.

Another drawback is the throughput of wafers. When, for example, asingle tank processor is used, switching from one liquid to anotherliquid takes time and the system can have a lower throughput compared toa classical wet bench approach.

Another drawback of the prior art cleaning or etching tools and methodsis that a non-uniformity in the cleaning or etching process occursremains. This is to be understood as follows: when a wafer is dipped ina tank, the bottom part of the wafer is always remaining some longer inthe cleaning or etching liquid than the upper part of the wafer.Therefore, the contact-time of several parts of the wafer is notidentical.

A further drawback of the prior art cleaning or etching tools andmethods is that a drying and/or rinsing step is required after acleaning or etching step. This drying and/or rinsing step takes extratime and therefore increases the processing time of wafers.

AIM OF THE INVENTION

It is an aim of the present invention to provide an apparatus and amethod for wet cleaning or etching a flat substrate wherein theabove-mentioned drawbacks of the prior art tools and methods have lessimpact. The apparatus and the method of the present invention allow ahigh throughput of flat substrates and a high cycle time, allow a moreuniform contact time for several parts of one wafer, allow thedevelopment of manufacturing systems with a small footprint and allowflat substrate manufacturing with the wet cleaning or etching step andthe required drying procedure being executed in one processing step.

SUMMARY OF THE INVENTION

The first object of the present invention is to provide an apparatus forwet cleaning or etching a flat substrate which comprises a tank with anopening for said substrate, said tank containing a cleaning or etchingliquid and being installed in an environment consisting essentially of agas or of a mixture of gases, such as a clean-room.

The said opening is present below the liquid-surface. The apparatusfurther comprises means to prevent the liquid from flowing through theopening out of the tank into the environment.

Advantageously, the substrate is transferred from within saidenvironment into said apparatus through said opening. Said apparatus canbe for instance used as part of a manufacturing line for the productionof said flat substrate or as part of a manufacturing line for developingfeatures in said substrate, such as integrated circuits in a siliconwafer.

In a first aspect of the apparatus according to the invention, saidmeans are realized by the dimensions of the opening, said opening beingsuch narrow that the liquid is prevented from flowing through it due tothe surface tension and/or the capillary effect of said liquid.

A first embodiment of the apparatus to achieve this is characterized inthat the opening is marginally larger than the thickness of saidsubstrate and is, with respect to the thickness of the wall of thetanks, a lengthened passage.

A second embodiment of the apparatus to achieve this is characterized inthat the opening is marginally larger than the thickness of saidsubstrate and is towards the environment a converging passage.

In a second aspect of the present invention, said means comprise aportion in the tank, which portion is above the liquid and is filledwith a gas or a gas mixture with a pressure being lower than the gaspressure within said environment. An advantageous embodiment of theapparatus to achieve this, is characterized in that said means comprisea pump connected with the room in the tank for sucking the gas andthereby reducing the gas-pressure in the tank and subsequently reducingthe liquid-pressure near the opening.

In a third aspect of the apparatus of the invention, said means comprisea room adjacent to said tank, said room having an opening for saidsubstrate and said room being filled with a gas or a gas mixture with apressure being higher than the pressure within said environment. Thewafer is transferred from said environment through said room into saidtank.

In a fourth aspect of the apparatus according to the invention, saidtank can be adjacent to a process chamber wherein, for instance, a gasphase etching process of semiconductor wafers can occur. In thisembodiment, wafers are transferred from within said environment throughsaid tank into said process chamber. In this way, the process chambercan be isolated from the environment thereby avoiding the introductionof moisture, oxygen or other rest gases in the chamber. Also, theintroduction of a wafer into the process chamber through said tank canoccur with a relatively simple automation setup. By preference, one tankis located adjacent to the process chamber at the inlet opening of saidprocess chamber and another tank is located adjacent to the processchamber at the outlet opening of said process chamber. Said processchamber can be an etcher or a RTP-system or a EPI-system or anothersystem being part of the manufacturing line of said substrate.

In a fifth aspect of the apparatus of the invention, said means comprisemeans outside the tank for directing a gas flow towards the opening. Inan advantageous embodiment of the invention, the gas flow comprises N₂.In another embodiment the gas flow comprises a vapor which is notcondensing on said substrate at said opening, said vapor being chosenfrom a group of substances which are miscible with said liquid and whichwill form with said liquid a mixture having a surface tension lower thanthat of said liquid alone. Preferably the vapor is an IPA vapor.

In a further aspect of the apparatus of the invention, said apparatuscomprises an inlet opening and an outlet opening. Said inlet opening canbe a slice in a first sidewall of said tank and said outlet opening canbe in another wall of said tank, said substrate being passed essentiallyhorizontally through said apparatus. In another embodiment said inletopening is a slice in the bottom of said tank and said outlet opening isin the top of said tank, said substrates being passed essentiallyvertically through said apparatus.

Yet another embodiment of the apparatus according to the invention ischaracterized in that the tank further comprises an inlet and an outletfor said liquid, said outlet being near said opening and said meanscomprises a pump connected to the outlet for sucking the liquid andthereby reducing the liquid-pressure near the opening.

The invention furthermore pertains as a second object to a method of wetcleaning or etching a flat substrate in particular a semiconductorwafer, comprising the steps of putting a substrate via an opening in andout of a tank filled with a cleaning or etching liquid. The method inaccordance with the invention is characterized in that the substrate isput into and/or taken out of the cleaning or etching liquid at a levelunderneath the liquid-surface via an opening in the tank present belowthe liquid-level in the tank and thereby preventing the liquid fromflowing through the opening out of the tank.

More particularly, the method of wet cleaning or etching of a flatsubstrate in a tank filled with at least one cleaning or etching liquidcomprises the steps of:

loading from within a gaseous environment said substrate through aninlet opening in said tank, said inlet opening being at a levelunderneath the surface of said liquid; and

unloading said substrate out of the tank into said gaseous environmentwhile preventing said liquid from flowing out of said tank.

An embodiment of the method according to the invention is characterizedin that, gas present above the liquid in the tank is sucked out of thetank to reduce the pressure inside the tank.

A further embodiment of the method according to the invention ischaracterized in that the liquid is sucked out of the tank near saidopening to reduce the liquid-pressure near the opening.

Another embodiment of the method according to the invention ischaracterized in that a gas flow is directed towards said opening fromoutside the tank to dry the substrate and to prevent the liquid fromflowing out of the tank.

The method of wet cleaning or etching a flat substrate comprises thestep of transferring from within a gaseous environment said substratethrough a tank filled with a cleaning or etching liquid, said substratebeing introduced into said tank through an inlet opening and taken outof said tank through an outlet opening underneath the liquid-surface andare transferred at a uniform speed through said liquid thereby exposingessentially each part of said substrate to said liquid for the same timeperiod.

Said apparatus and said method offer as an advantage the possibility ofplacing tanks no longer side by side but on top of each other. Thisapproach makes it possible to reduce drastically the footprint of acleaning or etching system and keeping a high throughput. Also acleaning tank can be placed directly besides of a next apparatus forfurther processing. The footprint of the apparatus of the inventionfurthermore is much smaller than the footprint of a classical wet bench.At the same time the apparatus and method disclosed in the presentapplication allow for keeping a high throughput. Said apparatus and saidmethod offer as another advantage the possibility of achieving a uniformcleaning or etching of wafers being transferred through the cleaning oretching liquid in said tank. As wafers can be transferred at a uniformspeed throughout said tank, each part of said wafers is exposed for thesame time period to said cleaning or etching liquid. Another advantageof said apparatus and said method is that by the use of dilute chemicalmixtures as a cleaning liquid in said tank, a rinsing step after saidcleaning or etching step in said tank can be avoided. In this way, afast processing time of wafers and a high throughput can be achieved. Itis disclosed in the prior art that if a cleaning solution is diluteenough, being diluted preferably with DI-water, wafers may be driedimmediately without ultra pure water rinsing after cleaning or etching.As a result, no detrimental residues are left behind on the wafersurface. This works well for instance with dilute HCl being diluted withDI-water in the range of pH 2-4. Another possibility is to have a verydilute SCl cleaning step with a volume mixing ratio of NH₄OH/H₂O₂/H₂O ina range of about 1/1/1000. Another possible solution that can be used toachieve this purpose is to use ozonated water. A detailed description ofthe use of such dilute chemicals for wafer cleaning/rinsing purposes isgiven in the co-pending patent application EP 96309145.9 “Drying ofWafers Directly From Chemical Baths with No Ultra-Pure Water Rinse”being co-owned by IMEC and Texas Instruments.

It is to be understood that the apparatus and method disclosed in thisdocument can be used for cleaning or etching of any kind of flatsubstrates. Thus, flat panel display substrates or solar cell substratesor glass substrates or metal plates or printed circuit boards can becleaned or etched using a specific embodiment of the apparatus accordingto this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further elucidated on the basis of the exampleembodiments of the apparatus and method according to the invention, asdepicted in the drawings. To this end:

FIG. 1 provides a vertically cross-section view of a first embodiment ofthe apparatus according to the invention.

FIG. 2 shows a horizontal cross-section of the apparatus according tothe first embodiment.

FIG. 3 shows a vertical cross-section of a detail of the apparatus nearan opening according to the first embodiment.

FIG. 4 shows another vertical cross-section of a detail of the apparatusnear an opening.

FIG. 5 provides a vertical cross-section view of a second embodiment ofthe apparatus according to the invention according to the secondembodiment.

FIG. 6 shows a horizontal cross-section of the apparatus according tothe second embodiment.

FIG. 7 shows the situation using another gas.

FIG. 8 provides a vertically cross-section view of a third embodiment ofthe apparatus according to the invention.

FIG. 9 provides a vertically cross-section view of a fourth embodimentof the apparatus according to the invention.

FIG. 10 shows a different mounting of the first embodiment of theapparatus.

FIG. 11 shows a specific embodiment of the apparatus of the presentinvention with a room at overpressure adjacent to said apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of the apparatus according to theinvention for wet cleaning or etching of flat substrates, in particularsemiconductor wafers. The apparatus 1 comprises a tank 3 with an inletand an outlet opening 5, 7 for a substrate 9. The tank 3 containingcleaning or etching liquid 11 and being installed in a gaseousenvironment 13, for instance, a clean-room. The openings are presentbelow the liquid-surface 15. The openings 5, 7 are such narrow that theliquid 11 is prevented from flowing through it due to the surfacetension and/or the capillary effect of said liquid. FIG. 2 shows ahorizontal cross-section of the apparatus with the substrate 9 partlyinserted in the tank 3 and sliding through the apparatus. At both sidesthe tank 3 contains guiding slits 17, 19 for the substrate transport.

In FIGS. 3 and 4 improved shapes of the openings are shown for maximumprevention of leakage. In the embodiment of FIG. 3 the opening 5 has atilted angle 21 and converges towards the environment 13. The opening ismarginally larger than the thickness of the substrate. In the embodimentof FIG. 4 the opening 5 is lengthened with respect to the thickness ofthe wall of the tank 3.

The form of the meniscus 23 will also depend on the hydrophobicity ofthe tank wall material. It is possible that the tank wall material hasto be treated in such a way that the contact angle versus the liquid,i.e. the surface tension of the material, has to stay stable during theoperation of loading a flat substrate 9 in the tank 3.

Advantageously, the tank wall is made of a material that increases thecapillary effect of said liquid. Such material can be a plastic.

The tank 3 comprises a portion 25 above the liquid 11 as can be seen inFIG. 1. This portion 25 is filled with a gas or a gas mixture with apressure being lower than the pressure within the environment 13. Tomaintain this reduced pressure a pump 27 may be connected with theportion 25 for sucking the gas and thereby reducing the gas-pressure inthe tank and subsequently reducing the liquid-pressure near theopenings. This further prevents the liquid from flowing through theopenings 5, 7 out of the tank 3. It is advantageous to regulate thepressure such that no air bubbles are sucked through the openings. Thepressure in the portion 25 can be for instance 2 to 3 millibar below thepressure of the environment 13. The preferred pressure in the portion 25is depending on the height of the liquid level above the openings 5, 7.The pressure advantageously is regulated such that the liquid in thetank is impeded from flowing out of the tank.

FIG. 5 shows a second embodiment of the apparatus according to theinvention. In this embodiment the apparatus 31 contains means 33 outsidethe tank 35 for directing a gas flow 37 towards the opening 39 to drythe substrate 41 and preventing the liquid 43 from flowing out of thetank 35. It is also possible to have further means 45 near the outletopening 47. In the situation shown in FIG. 6 an IPA vapor is used. Themeans can comprise a tube 49 blowing a vapor comprising IPA possiblymixed with another gas. The inlet and/or outlet atmosphere of the tankcan be saturated with said vapor. Using the vapor flow 51 over theoutlet opening of the tank, the substrate 41 can be taken out dry. TheMarangoni effect is used to change the surface tension at the opening.

In the situation shown in FIG. 7, N₂ is used instead of the IPA vapor.One can observe a drying effect at the blow itself. So, one will havethe choice to dry the substrate or with the force of a N₂ blow, or withthe marangoni effect, or with a combination of both effects.

Another way to improve the apparatus of the invention is to make use ofthe capillary force to reduce the pressure at the inlet and outletopenings. This can be done by making use of vertical partitions whichcreate a capillary force on the liquid by which the liquid is kept inthe tank. In FIG. 8 this is shown in a third embodiment of the apparatusaccording to the invention. Inside the tank 55 there are partitions 57to increase the capillary force and thereby reducing the liquid-pressurenear the openings 59, 61.

A modification to this is the use of a tank of capillary dimension ofonly a few millimeters width (not shown in the drawings). The substrateis processed in a tank and a line of only a few millimeters width ofliquid will touch the substrate. Another modification to this is the useof a tank with a height of only a few millimeters so that the liquidstays in by capillary force.

Another possibility is to make use of a pump to create a smalldifferential pressure at the outlet and inlet openings towards theenvironment. This is shown in FIG. 9. The tank 61 comprises an inlet 63and an outlet 65 for the cleaning or etching liquid 67. Vertical plates69 are inserted to guide the flow as shown by the picture. A pump 75 isconnected to the outlet for sucking the liquid and thereby reducing theliquid-pressure near the opening. When the plates 69 are sufficientlynear the substrate 77 the velocity of the liquid will be sufficientlyhigh close to the substrate surface to improve the supply of freshsolution.

In the above embodiments the inlet opening is a slice in the sidewall ofthe tank, the outlet opening is in another wall of the tank and thesubstrates being passed essentially horizontally through the apparatus.It is possible that only the outlet opening is under the liquid level,the outlet opening thus being in contact with the liquid and the inletopening not being in contact with the liquid. It is also possible (notshown in the drawings) to make the inlet opening as a slice in thebottom of the tank and the outlet opening in the top of the tank,whereby the substrates being passed essentially vertically through theapparatus.

As shown in FIG. 11, another embodiment of the apparatus according tothe invention has a room 81 adjacent to said tank 80, said room 81having an opening for said substrates and said room 81 being filled witha gas or a gas mixture with a pressure being higher than the pressurewithin said environment, the pressure above said liquid in the tank 80being equal to or lower than the pressure of said environment. Theopening of said room 81 advantageously is at the same height as theopening of said tank 80.

A connection 82 is provided on said room 81 to allow making anoverpressure in the room 81, preferentially by means of a gas flow beingheld at a pressure higher than the pressure of said environment.

Although the invention has been elucidated on the basis of theaccompanying drawings in the discussion to this point, it should benoted that the invention is in no way restricted to just thoseembodiments depicted in the drawings. The invention also encompasses allthe derivative embodiments which differ from the depicted embodimentswithin the scope defined in the claims. As an example, it is alsopossible to improve the inlet of the tank and facilitate the inlet ofthe substrate in the tank by tilting the device. This is shown in FIG.10. This gives the potential that at the inlet no marangoni dry or otherforce will be necessary while placing the substrate in the device.

What is claimed is:
 1. A method of wet cleaning or etching a flatsubstrate in a tank filled with at least one cleaning or etching liquid,said method comprising the steps of: loading within a gaseousenvironment said substrate through an inlet opening in said tank, saidinlet opening being at a level underneath the surface of said liquid;unloading said substrate out of the tank into said gaseous environment;and at least a part of time when loading and unloading said substrate,creating a pressure difference between inside and outside said tank sothat pressure inside said tank is lower than pressure outside said tankin order to inhibit liquid from flowing out of said tank.
 2. The methodof claim 1, wherein a gas flow is directed towards said opening fromoutside the tank, and wherein said gas flow dries the substrate andprevents the liquid from flowing out of the tank.
 3. A method of wetcleaning or etching a flat substrate in a tank filled with at least onecleaning or etching liquid, said method comprising the steps of: loadingwithin a gaseous environment said substrate through an inlet opening insaid tank, said inlet opening being at a level underneath the surface ofsaid liquid; and unloading said substrate out of the tank into saidgaseous environment while preventing said liquid from flowing out ofsaid tank, wherein a gas is present above the liquid in the tank, saidgas being sucked out of the tank to reduce the pressure inside the tank.4. A method of wet cleaning or etching a flat substrate comprising:transferring said substrate through a tank filled with a liquid, saidsubstrate being introduced into said tank through an inlet opening andtaken out of said tank through an outlet opening underneath theliquid-surface while preventing said liquid from flowing out of saidtank, wherein said substrate is introduced into the tank from a gaseousenvironment and transferred at a uniform speed through said liquid,thereby exposing each part of said substrate to said liquid foressentially the same period; and at least a part of time whentransferring said substrate, creating a pressure difference betweeninside and outside said tank so that pressure inside said tank is lowerthan pressure outside said tank in order to inhibit liquid from flowingout of said tank.
 5. A method of wet cleaning or etching a flatsubstrate comprising the step of transferring said substrate through atank filled with a liquid, said substrate being introduced into saidtank through an inlet opening and taken out of said tank through anoutlet opening underneath the liquid-surface while preventing saidliquid from flowing out of said tank, wherein said substrate isintroduced into the tank from a gaseous environment and transferred at auniform speed through said liquid, thereby exposing each part of saidsubstrate to said liquid for essentially the same period, wherein a gasis present above the liquid in the tank, said gas being sucked out ofthe tank to reduce the pressure inside said tank.
 6. A method for wetcleaning or etching a flat substrate, the method comprising the stepsof: providing a tank with an opening in said tank, wherein said tankcontains at least one cleaning or etching liquid and is installed in anenvironment consisting essentially of a gas or of a mixture of gases;transferring at least a portion of the flat substrate into or out ofsaid tank; the opening being below the liquid surface; and directing agas flow towards said opening to inhibit said liquid from flowingthrough said opening out of said tank into said environment, the step ofdirecting a gas flow being performed at least a part of time during thestep of transferring at least a portion of the flat substrate into orout of said tank.
 7. The method of claim 6, wherein liquid is preventedfrom flowing through said opening out of said tank when transferring thesubstrate into and out of said tank.
 8. The method of claim 6, whereinsaid gas flow comprises N₂ as a constituent of the gas flow mixture. 9.The method of claim 6, wherein the step of transferring at least aportion of the flat substrate into or out of said tank includestransferring the substrate horizontally.
 10. The method of claim 6,wherein the step of transferring at least a portion of the flatsubstrate into or out of said tank includes transferring the substratevertically.
 11. The method of claim 6, wherein said tank is adjacent toa process chamber being part of a manufacturing line of said substrate;and wherein said step of transferring at least a portion of the flatsubstrate into or out of said tank includes transferring said substratefrom within said tank into said process chamber.
 12. A method for wetcleaning or etching a flat substrate, the method comprising the stepsof: providing a tank with an opening in said tank, wherein said tankcontains at least one cleaning or etching liquid and is installed in anenvironment consisting essentially of a gas or of a mixture of gases;transferring at least a portion of the flat substrate into or outof-said tank, the opening being below the liquid surface; and preventingsaid liquid from flowing through said opening out of said tank into saidenvironment during the step of transferring at least a portion of theflat substrate into or out of said tank, wherein inside the tank is afirst pressure and outside said tank is a second pressure, and whereinthe step of preventing said liquid from flowing through said opening outof said tank includes modifying the first pressure relative to thesecond pressure so that the second pressure is higher than the firstpressure.
 13. A method for wet cleaning or etching a flat substrate, themethod comprising the steps of: providing a tank with an opening in saidtank, wherein said tank contains at least one cleaning or etching liquidand is installed in an environment consisting essentially of a gas or ofa mixture of gases; transferring at least a portion of the flatsubstrate into or out of said tank, the opening being below the liquidsurface; and preventing said liquid from flowing through said openingout of said tank into said environment during the step of transferringat least a portion of the flat substrate into or out of said tank,wherein said tank includes a second portion, said second portion beingabove said liquid; and modifying pressure within said second portion sothat the pressure in said second portion is lower than pressure withinsaid environment.
 14. The method of claim 13, wherein the step ofmodifying pressure within said second portion includes removing gas insaid second portion to reduce the gas-pressure in said second portionand subsequently reduce the liquid-pressure near said opening.
 15. Amethod for wet cleaning or etching a flat substrate, the methodcomprising the steps of: providing a tank with an opening in said tank,wherein said tank contains at least one cleaning or etching liquid andis installed in an environment consisting essentially of a gas or of amixture of gases; transferring at least a portion of the flat substrateinto or out of said tank, the opening being below the liquid surface;and preventing said liquid from flowing through said opening out of saidtank into said environment during the step of transferring at least aportion of the flat substrate into or out of said tank, wherein the stepof preventing includes directing a gas flow towards said opening, andwherein said gas flow comprises a vapor which does not condense on saidsubstrate at said opening, said vapor being chosen from a group ofsubstances which are miscible with said liquid and which will form withsaid liquid a mixture having a surface tension lower than that of saidliquid alone.
 16. The method of claim 15, wherein the vapor comprisesIPA.
 17. A method for wet cleaning or etching a flat substrate, themethod comprising the steps of: providing a tank with an opening in saidtank, wherein said tank contains at least one cleaning or etching liquidand is installed in an environment consisting essentially of a gas or ofa mixture of gases; transferring at least a portion of the flatsubstrate into or out of said tank, the opening being below the liquidsurface; and preventing said liquid from flowing through said openingout of said tank into said environment during the step of transferringat least a portion of the flat substrate into or out of said tank,wherein said substrate is a semiconductor wafer and said environment isa clean room.
 18. A method for wet cleaning or etching a flat substrate,the method comprising the steps of: providing a tank with an opening insaid tank, wherein said tank contains at least one cleaning or etchingliquid and is installed in an environment consisting essentially of agas or of a mixture of gases; transferring at least a portion of theflat substrate into or out of said tank, the opening being below theliquid surface; and at least a part of time when transferring saidsubstrate, creating a pressure difference between inside and outsidesaid tank so that pressure inside said tank is lower than pressureoutside said tank in order to inhibit liquid from flowing out of saidtank.
 19. A method for wet cleaning or etching a flat substrate, themethod comprising the steps of: providing a tank with an opening in saidtank, the opening having a thickness, wherein said tank contains atleast one cleaning or etching liquid having a uniform depth of a fewmillimeters and is installed in an environment consisting essentially ofa gas or of a mixture of gases; transferring at least a portion of theflat substrate into or out of said tank, the opening being below theliquid surface, wherein said depth of said liquid inhibiting said liquidfrom flowing through said opening out of said tank.
 20. A method for wetcleaning or etching a flat substrate, the method comprising the stepsof: providing a tank with an opening and a plurality of partitions insaid tank, wherein said tank contains at least one cleaning or etchingliquid and is installed in an environment consisting essentially of agas or of a mixture of gases; transferring at least a portion of theflat substrate into or out of said tank, the opening being below theliquid surface, wherein said partitions reduce liquid pressure near theopening inhibiting said liquid from flowing through said opening out ofsaid tank.